You won't see your doctor, you'd see a dentist. Chances are it's a blocked saliva gland.  Try that lemon stuff you get in those wee plastic bottles. Don't drink it, just swish it about in your mouth. Lemons cause you to produce a lot of saliva.  If that doesn't shift it, you'll have to go to a dentist because they can get painful quickly( like very, very painful  - a 10/10 on the pain scale) if it blocks the entire gland. Or gets infected.  Does it bother you after eating?

Blocked saliva glands tend to be caused by stones. Hence the hardness.